1. Field of the Invention
This invention relates to a voltage detecting apparatus, especially a voltage detecting apparatus for detecting a voltage of a direct current power source.
2. Description of the Related Art
As a voltage detecting apparatus for detecting a voltage of a direct current power source, there is generally, for example, a flying-capacitor-type insulation detecting apparatus. For detecting an insulating condition of an insulation condition of the high-voltage direct-current power source, a value of earth-fault resistance is calculated based on a measured value of a voltage between both terminals of an ungrounded capacitor (i.e. flying capacitor) charged with a voltage of the high-voltage power source and a measured value of a voltage between both terminals of the capacity, one terminal of which is grounded through a resistor, similarly charged with the voltage of the high-voltage power source. Thereby, the insulation condition of the high-voltage power source can be detected. There are Patent documents 1, 2 as references.
FIG. 4 is a circuit diagram showing one example of structures of general insulation detecting apparatuses by prior art. In FIG. 4, a mark V is a high-voltage power source (a direct-current power source), which is formed by connecting N numbers of batteries in series, and insulated from an earth G in a low-voltage system, such as a micro-computer 10.
The insulation detecting apparatus, as shown in FIG. 4, includes a bipolar capacitor C, a first switch SW1 for connecting a positive electrode of the high-voltage power source insulated from the earth G to one terminal of the capacitor C, and a second switch SW2 for connecting a negative electrode of the high-voltage power source insulated from the earth G to the other terminal of the capacitor C.
The micro-computer 10 has a function of measuring a voltage supplied to an input port A/D by converting the voltage from analog data to digital data. The insulation detecting apparatus includes a third switch SW3 for connecting the one terminal of the capacitor C to the input port A/D, and a fourth switch SW4 for connecting the other terminal of the capacitor C to the earth G.
The insulation detecting apparatus also includes a first resistor R1 provided between a terminal near to the input port A/D of the third switch SW3 and the earth G, and a second resistor R2 provided between a terminal near to the earth G of the fourth switch SW4 and the earth G.
To the input port A/D, the voltage is supplied through a protection circuit 11. The protection circuit 11 includes a protection resistor Rp1 provided between a terminal near to the third switch SW3 of the first resistor R1 and the input port A/D, and a clamp diode Dc provided between a terminal near to the input port A/D of the protection resistor Rp1 and the earth G.
The protection resistor Rp1 performs as a current limiting resistor to prevent over-current flowing into the input port A/D of the micro-computer 10. It can be prevented by the clamp diode Dc that too large positive or negative voltage to damage the micro-computer 10 is supplied to the input port A/D.
The insulation detecting apparatus also includes a resistor select circuit 12 provided between a connecting line of the first switch SW1 and the third switch SW3 and the capacitor C. The resistor select circuit 12 is structured by connecting in parallel a series circuit, which has a first diode D1 connected in a forward direction from the connecting line of the first switch SW1 and the third switch SW3 toward the capacitor C and a first select resistor Rc1, and a series circuit, which has a second diode D2 connected in a reverse direction opposite to the direction of the first diode D1 and a second select resistor Rc2.
An optical MOSFET is used for the first, second, third or fourth switches SW1-SW4 so as to be controllable by the micro-computer and insulate the high-voltage power source V. In a reset circuit 13, by controlling a reset switch SWr to be close, electric charge stored in the capacitor C is rapidly discharged through a discharge resistor Rdc.
Actions of the insulation detecting apparatus structured above will be described with reference to a flowchart in FIG. 5. In step S101, the micro-computer 10 measures a voltage between both-terminals V0 of the high-voltage power source. Process of measuring this is proceeded as followings. When all switches are open in an initial condition, the micro-computer 10 controls the first switch SW1 and the second switch SW2 to both be closed for charging the capacitor C with the voltage of the high-voltage power source V.
After controlling the first and second switches SW1, SW2 to be open, by controlling the third and fourth switches SW3, SW4 to both be closed, a voltage between both terminals of the capacitor C, that is the voltage between both-terminals V0 of the high-voltage power source, is supplied into an input port A/D of the micro-computer 10. Thereby, the voltage between both-terminals V0 is inputted as a voltage of the high-voltage power source V into the micro-computer 10.
In step S102, the micro-computer 10 measures a voltage VRL− corresponding to a value of a negative-electrode-side earth resistor RL−. This measuring is proceeded as following. After the reset circuit 13 resets, the micro-computer 10 controls the first and fourth switches SW1, SW4 to both be closed. Then, the capacitor C is charged with a charged voltage corresponding to a resistance value of the negative-electrode-side earth resistor RL−.
After controlling the first switch SW1 to be open, the micro-computer 10 closes the third and fourth switches SW3, SW4. Thereby, the voltage between both-terminals of the capacitor C, that is the voltage VRL− corresponding to the resistance value of the negative-electrode-side earth resistor RL− is inputted into the micro-computer 10.
In step S103, the micro-computer 10 measures a voltage VRL+ corresponding to a resistance value of the positive-electrode-side earth resistor RL+. This measurement proceeds as follows. After the reset circuit 13 resets, the micro-computer 10 controls the second and third switches SW2, SW3 to both be closed. Thereby, the capacitor C is charged with a charged voltage corresponding to a resistance value of the positive-electrode-side earth resistor RL+.
After controlling the second switch SW2 to be open, the micro-computer 10 closes the third and fourth switches SW3, SW4. Thereby, the voltage between both-terminals of the capacitor C, that is the voltage VRL+ corresponding to the resistance value of the positive-electrode-side earth resistor RL+ is inputted into the micro-computer 10.
In step S104, the micro-computer 10 performs a calculation (VRL−+VRL+)V0 of dividing a sum of the measured voltage VRL− corresponding to the resistance value of the negative-electrode-side earth resistor RL− and the measured voltage VRL+ corresponding to the resistance value of the positive-electrode-side earth resistor RL+ by the measured voltage V0 corresponding to the voltage of both terminals of the high-voltage power source V. In step S105, the micro-computer 10 calculates a value of the earth resistance of the high-voltage power source V by the above calculated value with reference to a conversion table of calculated values to values of the earth resistances pre-stored in an inner memory.
Thus, whenever the capacitor C is charged with the voltage of both terminals V0 of the high-voltage power source V, the voltage VRL+ corresponding to the resistance value of the positive-electrode-side earth resistor RL+ and the voltage VRL− corresponding to the resistance value of the negative-electrode-side earth resistor RL−, by controlling the first-fourth switches SW1-SW4 respectively, and reading the voltage between both terminals of the capacitor C, the micro-computer 10 can detect an insulation condition of the high-voltage power source V. Refer Patent documents of Japan Published Patent Applications No. 2004-170103 and No. 2004-245632.